First what does "having no jams" really mean?

노잼 (No Jaem)” which stands for “재미없어 (Jaem Mi Oabsaw)” which means “Not Fun”. They use the slang 노잼 (No Jaem) when people try to say something and nobody think it’s funny.
As you can see in the video, before Rapmon said that Jimin got no Jams, Jimin was talking a bunch of nonsense, and then J-Hope says “Jimin is very no fun” which in direction translation to Korean means 재미없어 (Jaem Mi Oabsaw) and in Korean slang is 노잼 (No Jaem).
Here is the link to the whole resource.
Watch the video to see what's going on at 1:51 is where the madness begins.